1. Technical Field of the Invention
This invention generally pertains to analog signal processing circuits. In particular, this invention is directed to an improved input buffer circuit providing improved performance over a wider range of input signal levels.
2. Background Art
Analog signal processing circuits often require unity gain buffers at their inputs to minimize loading on the circuit which drives them. A method of implementing such a buffer is to use a single device unity-gain amplifier such as a source follower or common drain transistor. In such a configuration, the source is typically biased with a DC current of 10 to 100 .mu.A, the input signal is applied to the gate, and the output appears at the source, shifted by a voltage equal to the V.sub.GS of the device at the given bias level.
A problem occurs with this circuit because the shift voltage V.sub.GS is a function of the threshold voltage of the device. Since the device threshold is a function of its source-body voltage (i.e., is affected by the input signal), and the source voltage is the output signal, the level shift from input to output becomes a function of the input signal. Thus, this problem is inherent in the FET source-body structure and causes an attenuation of the output signal because V.sub.GS is proportional to the input signal, as shown in FIG. 1B (not drawn to scale). Ideally, the biasing means should compensate for the source-body voltage fluctuation and maintain V.sub.GS at a constant (DC) level without the variation seen in FIG. 1B.
Many analog circuits that use such conventional followers are differential circuits implemented to reject common-mode noise. In such a case the effect described above is doubled because the thresholds change in opposite directions. A typical differential source follower is illustrated in FIG. 1 and its transfer function shown in FIG. 2. Note the difference between the input signal voltage and the output signal voltage for signal magnitudes greater than about .+-.200 mV, which mismatch by about 5% in the .+-.500 mV range. For circuits that process the absolute amplitude of the input signal, such as peak detectors, this margin of error can be a significant drawback.
It is an object of the invention to provide a differential source follower with no body effect induced errors.
It is another object of the invention to provide an analog input buffer providing accurate performance over a wide range of input signal levels.
It is yet another object of the invention to provide a differential source follower with a low power mode.